Embodiments of the present invention relate to circuits, and more particularly, to digital-to-analog converters.
Digitally controlled current sources, or current sinks, are used in many types of circuits. An example of a digitally controlled current sink, 101, is shown in FIG. 1, comprising port (or node) 102 for receiving a bias voltage Vbias, port (or node) 104 for receiving a digital signal d, and port (or node) 106 in which a current Iout, is sunk. When port 104 is HIGH, nMOSFET (n Metal Oxide Semiconductor Field Effect Transistor) 112 is OFF and nMOSFET 108 is ON, so that the gate of nMOSFET 110 is at the bias voltage Vbias in order to sink current Iout. When port 104 is LOW, nMOSFET 108 is OFF and nMOSFET 112 is ON so that nMOSFET 110 is OFF and no current is sunk.
For convenience, throughout these letters patent, the term xe2x80x9ccurrent sourcexe2x80x9d is meant to include either a circuit that sources a current, a circuit that sinks a current, or both. Similarly, although a current source may source current and a current sink may sink current, for convenience both functions will be referred to as sourcing a current. It will be clear from context, such as a circuit drawing, whether a current is sourced or sunk. Consequently, the circuit of FIG. 1 may be referred to as a current source, where current Iout is sourced at port or node 106.
A current mode digital-to-analog (D/A) converter may employ a plurality of digitally controlled current sources to convert a digital signal to an analog signal. These current sources may be connected in parallel. For example, in FIG. 2 n digitally controlled current sources are connected in parallel to node 202, which is connected to network 204. For each i=0, 1, . . . , nxe2x88x921, the digitally controlled current source indexed by i sources a current Ii. An output signal may be taken at node 202. For example, if network 204 is a simple resistor connected to a voltage source, the voltage at node 202 is an analog signal indicative of the digital signals controlling the digitally controlled current sources.
Other circuit functions may be realized by the high level functional diagram of FIG. 2 depending upon network 204. In general, network 204 represents a sub-circuit, and may comprise active elements as well as passive elements. For example, network 204 may be a differential amplifier in which the current sourced at node 202 provides biasing current to adjust the amplifier gain, where the gain is controlled by the digital signals controlling the digitally controlled current sources.
Some applications may require a relatively large number of parallel connected digitally controlled current sources. For example, a current mode D/A according to the circuit of FIG. 2 converter with a resolution of N bits uses 2N digitally controlled current sources. A large number of current sources connected in parallel to a node may lead to the node having a large capacitance, which may slow down the speed of the circuit.